Shaft seal and method for manufacturing it

ABSTRACT

A shaft seal seals a shaft in a hydraulic steering valve. The seal is a sealing ring surrounding the shaft. The sealing ring has a sealing edge contacting the outer surface of the shaft to form a seal. The sealing edge extends circumferentially around the outer surface of the shaft in a sinusoidal fashion. The sealing ring has first and second surfaces which extend radially outward from the sealing edge and which diverge relative to each other as they extend radially outward from the sealing edge. The first and second surfaces extend entirely circumferentially around the outer surface of the shaft. The first surface extends at a constant first angle relative to said sealing edge throughout the entire circumferential extent of the first surface and the second surface extends at a constant second angle relative to the sealing edge throughout the entire circumferential extent of the second surface.

FIELD OF THE INVENTION

The invention relates to a shaft seal, particularly a seal for the shaftof a hydraulic steering valve, having a sealing ring zone surroundingthe shaft and, located on the shaft side of the sealing ring zone, atleast one sealing edge which contacts the shaft to form a seal in theregion of an annular sealing surface defining a radial plane which liesessentially at right angles to the longitudinal center axis of theshaft, wherein the sealing edge, of which there is at least one, has acontour such that it intersects the annular sealing surface severaltimes. The invention furthermore relates to a method for manufacturingsuch a shaft seal.

DESCRIPTION OF RELATED ART

Generic shaft seals of the described type are known per se. One of thepreferred applications for such seals is in hydraulic steering valves.Hydraulic steering valves, such as rotary disk valves, are known per se.An input shaft and an output shaft that is connected with the inputshaft by means of a torsionally elastic element, typically a torque rod,are arranged in a housing. The output shaft engages with the rack of asteering gear via a pinion. Normally, a valve sleeve surrounding theinput shaft is connected with the output shaft. Hydraulic fluid issupplied to the corresponding steering ducts through steering bores viaconnections disposed on the side the housing.

In this type of power-assisted steering valves, shaft seals of theaforementioned type are used. Such seals typically comprise an outersealing ring zone which is made in such a way that the shaft seal can befixed relative to the shaft in its housing or to another component.Typically, the sealing ring zone is made of an elastic material with anannular embedding for reinforcement which can also be an elasticelement. Normally, at least one sealing edge, which forms a singlecomponent together with the elastic material, is disposed on the sealingring zone pointing toward the shaft. In practice, this sealing edgeforms a ring surrounding the shaft, which is usually very small in axialdirection, i.e., it takes the form of a ridge. There is typically nodrag flow in the direction of the sealing edge, and thus the sealsurface, as a function of the system. The seal surface is an imaginaryannular surface along the outer contour of a shaft along which thesealing edge contacts the shaft to form a seal. In the absence of dragflow, this may result in inadequate lubrication of the seal surface,particularly in fast rotating shafts. This significantly increasesmechanical and thermal stress which is due, among other things, to thefact that there is no fluid exchange in the seal surface or the sealgap. Normally, the sealing edge lies in a plane perpendicular to theshaft axis. Here, lubrication depends on complicated flow processes dueto the interaction between microscopic roughnesses and the viscoelasticseal material. To improve lubrication in rotating shafts underunfavorable lubricant flow conditions, it has previously been proposedto arrange an O-ring at an angle to the shaft axis. The use of an O-ringlying at an angle to the shaft axis produces additional friction andalso does not permit tolerances, that is, no eccentricity compensationis possible. In fast turning shafts, the seals float and lubricationoccurs as a result of what is known as hydrodynamic friction. However,this does not occur with slow moving shafts, for example, those used inpower-assisted steering systems.

Characteristic for the steering valve application is the lack oflubrication between the shaft and the seal which occurs because of theslow speeds and the lack of drag flow in the contact area of shaft andseal. This can create undesirably high friction and result in stick slipphenomena including noise.

DE 22 64 042 A1 discloses a shaft seal for fast moving shafts. This sealis made in such a way that the sinusoidal seal edge is formed when theseal portion made of elastic material is manufactured. Since the sealingedge sinusoidally migrates in axial direction and should have a constantdiameter in order to create constant pressure conditions, the angles oneither side of the sealing edge element are continuously changing. Thisnegatively influences the sealing mechanism which is substantiallydetermined by the two angles. DE 38 30 706 A1 also discloses a shaftseal for fast rotating shafts. Here, the sealing edge is also formed insuch a way that it extends sinusoidally in axial direction whilecreating a constant diameter, but the sealing edge angles are keptconstant on both sides around the entire circumference. The sealing edgeis not very high and capillary active clearance spaces are formed onboth sides of the sealing edge. These are used to maintain adequatelubrication of the sealing element which is intended for fast rotatingshafts. While constant sealing edge angles on both sides are requiredfor generic shaft seals to obtain a defined sealing characteristic, thecapillary active clearance spaces on both sides of the sealing edge aredisadvantageous for the pressure sealing of slow moving shafts,particularly shafts of steering gears. This is because here, in contrastto the application in fast rotating shafts, the area of the seal surfacedoes not lift or float but instead, when pressure is acting on the shaftseal, which normally does not occur but is the case here, the area ofthe seal surface is substantially pressed against the shaft. Anycapillary active clearance spaces are eliminated when pressure is actingand the contact area between seal and shaft increases substantially.Thus, lubrication in this area decreases, the intended drag flow can nolonger take place and, as a result, friction, wear and noise increase.

SUMMARY OF THE INVENTION

Based on this state of the art, it is the object of the invention toimprove a generic type shaft seal to permit improved lubrication of theseal surface in order to reduce frictional torques, noise and/or stickslip phenomena.

It is a further object of the invention to define a method formanufacturing such a shaft seal.

To attain this object with respect to the manufacturing process, theinvention proposes a method for manufacturing a shaft seal, particularlyfor shafts of a hydraulic steering valve, with a sealing ring zonesurrounding the shaft and, located on the shaft side of the sealing ringzone, at least one sealing edge which contacts the shaft to form a sealin the region of an annular sealing surface defining a radial planewhich lies essentially at right angles to the longitudinal center axisof the shaft, the sealing edge having a shape such that it intersectsthe annular sealing surface several times. The sealing edge has aconstant inside diameter and pressure angles which are constant on bothsides over the entire circumference and is formed on a cylindrical basewhich is made on the inner circumference of the sealing ring zone with awidth corresponding at least to the width of the axial extent of thesealing edge. The cylindrical base in the area of the adjacent surfacesnext to the sealing edge is designed so that, from the interior of thesealing edge, it is at least partially spaced apart in such a way thatthese surfaces cannot contact the shaft when pressure is acting.

The manufacturing method according to the invention makes it possible onthe one hand to produce a sealing edge with constant pressure angles onboth sides over the entire circumference, which need not necessarily besymmetrical, and on the other hand to prevent the occurrence ofcapillary active zones or at least to reduce their negative effects. Dueto the pressures occurring in steering valves, capillary active zonescause the regions adjacent to the sealing edge to be pressed against theshaft if pressure is acting, which increases friction and wear. Bydesigning the clearance spaces in the context of the manufacturingmethod according to the invention, the pressure effects are reduced suchthat the seal produced according to the invention has a very goodservice life and, furthermore, ensures adequate lubrication.

The manufacturing method according to the invention simplifies themanufacturing process and makes production economical and efficient.This production process can start with a cylindrical base surface inwhich the sealing edge can then be formed in known manner. Thecylindrical surfaces adjacent to the sealing edge must then be processedin such a way that the distance of the cylindrical areas is at leastpartially increased in relation to the center of the seal. The referenceto the center of the seal also corresponds to the reference to thecircumference of the sealing edge. Said design can be accomplished, forexample, by applying additional material. These areas can also remainfrom the start when the cylindrical surface is formed. The manufacturingmethod according to the invention prevents the cylindrical areasadjacent to the sealing edge from being pressed against the shaft whenpressure is acting.

Advantageously, all of the adjacent areas are completely recessed sothat capillary active zones are completely prevented. However, thisfails to increase the height of the sealing edge which would cause anunnecessary weakening of the sealing edge. The recesses can be madespherical, for example.

According to a particularly advantageous proposal of the invention,axially extending notches are made in the adjacent areas up to the edgearea.

To attain the aforementioned object with respect to the shaft sealitself, the invention proposes that the sealing edge have a constantangle on both sides over the entire circumference and the adjacentregions next to the sealing edge, viewed from the interior of thesealing edge, be at least partially spaced apart in such a way thatthese surfaces cannot contact the shaft when pressure is acting.

This technical solution has the effect that the viscous steering gearfluid adhering to the shafts to be sealed is dragged by thecorresponding shaft under the area of the seal surface extending inaxial direction which necessarily improves lubrication and thus reducesor prevents friction and/or stick slip phenomena caused by the seal.Noise is also automatically reduced.

This drag mechanism is particularly advantageous if, according to oneproposal of the invention, the contour of the sealing edge iswave-shaped in axial direction.

A further advantageous proposal is to form notches in the adjacent areaswhich extend in substantially axial direction. According to aparticularly advantageous proposal, the adjacent areas are hollowed out,preferably spherically, without thereby increasing the height of thesealing edge.

Also advantageous are radially extending indentations in the sealingedge. Such indentations promote drag flow into the sealing surface gapand thereby enhance lubrication. According to a particularlyadvantageous proposal, the shaft seal is given a wave shape in radialdirection.

The invention proposes the arrangement of a plurality of sealing edgesin the sealing ring zone. These sealing edges can be parallel or theycan alternate in relation to their wave shape such that differentlyoriented waves are arranged next to each other.

The invention provides a simple shaft seal which is economical toproduce and, when used in steering gears, particularly suitable toreduce noise and improve lubrication. The shaft seal according to theinvention may be used without increasing the axial structural space of ageneric rotary disk valve.

Advantageously, the invention proposes that a lip profile for trappingdirt be arranged on the sealing area. Such dirt trapping lip profilesare preferably disposed on the atmospheric side next to the sealingedges. Grease may be introduced between the dirt trapping lip profileand a sealing edge. This is particularly advantageous if, due to thesealing edge contour, the grease can be dragged into the area of theseal surface for lubrication.

Thus, the invention provides a particularly advantageous, novel rotarydisk valve with good shaft seal lubrication.

Additional advantages and features of the invention are illustrated bymeans of the figures and the following description.

DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic representation of an embodiment of the invention.

FIG. 1 shows a shaft seal 1 in a schematic arrangement which ispartially shown in cross section. Shaft seal 1 comprises a sealing ringzone 3, a reinforcing insert 5, a sealing lip 4 with sealing edgecylindrical area 11 and clamping ring 7.

Sealing edge 8 is given a wave shape in axial direction. Waves 9 havethe effect that lubricant collects in the wave indentations on thelubrication side which is then carried into the seal gap as a result ofthe rotational movement of shaft 2.

Reference number 10 designates the indentations formed in the adjacentareas. Next to sealing edge 8, whose height is exaggerated forillustration purposes, the manufacturing method according to theinvention creates flat adjacent areas, the capillary action of which isinterrupted by the introduction of indentations 10.

    ______________________________________                                        List of Reference Numbers:                                                    ______________________________________                                                   1   shaft seal                                                                2   shaft                                                                     3   sealing ring zone                                                         4   sealing lip                                                               5   reinforcement                                                             6   groove                                                                    7   clamping ring                                                             8   sealing profile                                                           9   wave                                                                      10  indentations                                                              11  cylindrical area                                               ______________________________________                                    

I claim:
 1. A shaft seal for sealing a shaft in a hydraulic steeringvalve, said shaft being centered on a longitudinal axis and having acylindrical outer surface, said shaft seal comprising:a sealing ringsurrounding the shaft; said sealing ring having a sealing edgecontacting the outer surface of the shaft to form a seal, said sealingedge extending circumferentially around the outer surface of the shaftin a sinusoidal fashion and forming said seal entirely around the outersurface of the shaft, said sealing ring having first and second surfaceswhich extend radially outward from said sealing edge and which divergerelative to each other as they extend radially outward from said sealingedge, said first and second surfaces extending entirelycircumferentially around the outer surface of the shaft, said firstsurface extending at a constant first angle relative to said sealingedge throughout the entire circumferential extent of said first surfaceand said second surface extending at a constant second angle relative tosaid sealing edge throughout the entire circumferential extent of saidsecond surface, said sealing ring having a first cylindrical portionprojecting axially away from said first surface in a first direction anda second cylindrical portion projecting axially away from said secondsurface in a second direction opposite to said first direction, saidfirst and second cylindrical portions being spaced radially from theouter surface of the shaft.
 2. The shaft seal of claim 1 wherein saidfirst and second cylindrical regions are completely hollowed out.
 3. Theshaft seal of claim 1 wherein said first cylindrical region includesnotch-shaped indentations to prevent capillary action.
 4. The shaft sealof claim 1 wherein second cylindrical region includes notch-shapedindentations to prevent capillary action.
 5. The shaft seal of claim 1wherein said sealing ring includes a dirt-trapping lip portion arrangedon an atmospheric side of said sealing edge and extending from saidfirst cylindrical portion of said sealing ring.
 6. The shaft seal ofclaim 5 wherein grease is introduced between said dirt-trapping lipportion and said sealing edge.
 7. The shaft seal of claim 1 wherein saidsealing ring is made of an elastic material and includes an annularreinforcing insert embedded in the elastic material of said sealingring.
 8. The shaft seal of claim 1 wherein said sealing ring includes anannular groove located radially outward of said sealing edge.
 9. Theshaft seal of claim 8 wherein said sealing ring includes an annularclamping ring disposed in said annular groove, said clamping ringexerting a radially inwardly directed clamping force on said sealingedge.